I have been to Fenway and Old Yankee Stadium (henceforth referred to as OYS) my fair share of times, and I don't think it has the same causes for concern that the OYS had. Granted, I'm sure that my bias is playing a factor, but my experiences at Fenway and the OYS were very different. First, the entire OYS reeked of urine. I swear to god, the pathways, the seats, everything. Also, there wasn't a food vendor in sight in the upper deck. The only "refreshment" being served was $8 cans of lukewarm Coors Light.
While I felt that the OYS wasn't very comfortable, I think that it could have lasted awhile longer. In other words, I don't think it was worth tearing down all of that history in order to make the ballpark even more "executive" or "holier than thou" than it already is. I felt that the tradition and history alone could have kept the ballpark running for awhile.
I feel similarly about Fenway. While I don't notice the urine smell that permeated the entire OYS, and while I find the seating better at Fenway (all the food makes its way around), I think that Fenway should still be in use for years to come. I think it's fine structurally as the renovations have brought it up a notch, and while the seating could be improved, that the experience at Fenway is worth the temporary discomfort.
Basically, my point is that Fenway should last awhile longer, but the Red Sox will eventually need a new ballpark. We know that Fenway can't last forever, but I don't feel that there's a good reason to tear it down soon and build another ballpark. Plus I'm sure that the last thing the country needs is to fund more money for the creation of another new ballpark.